Interconnection and packaging-related issues are among the factors that determine not only the number of circuits that can be integrated on a chip but also the performance of the chip. These issues have gained importance as advances in chip design have led to reduced sizes of transistors and enhanced chip complexity. The industry has come to realize that merely having a fast chip will not necessarily result in a fast system; the fast chip must also be supported by equally fast and reliable connections. Essentially, the connections, in conjunction with the packaging, supply the chip with signals and power and redistribute the tightly packed terminals of the chip to the terminals of a carrier substrate and then to a circuit board.
One example of such an integrated circuit device is known as a “flip-chip.” Flip-chip attachment generally includes electrically and mechanically attaching a semiconductor die by its active surface to an interposer substrate or other carrier substrate using an array of discrete conductive elements formed on the semiconductor die. The discrete conductive elements are formed and bonded to bond pads on the active surface of the semiconductor die, usually during fabrication of the semiconductor die along with a large number of others in wafer form, after which the wafer is singulated into the individual semiconductor die.
The discrete conductive elements usually are configured as minute conductive bumps or balls, but also may include studs, pillars or columns of various configurations. The conductive bumps or discrete conductive elements are typically, in the case of solder balls, attached to the bond pads by first forming an under bump metal (UBM) compatible with the material of the bond pads, as well as the solder balls. The UBM for solder balls to be placed on copper bond pads commonly includes nickel (Ni) and a thin layer of gold (Au). There can also be an intermediate palladium (Pd) layer in between the Ni and the Au forming an electroless nickel electroless palladium immersion gold (ENEPIG). A thin layer of gold (Au) may also be formed over the copper bond pads. The Ni of the UBM may be formed by an electroless plating process. A preformed solder ball (for example, 60% Sn and 40% Pb) may then be provided on the UBM and heated to a predetermined reflow temperature so as to bond the solder balls to the UBM structures on the wafer. Alternatively, a solder paste may be disposed on the UBM and then heated to liquify and form a solder ball.
As the size of the bond pad decreases, it becomes increasingly difficult to form the Ni material of the UBM by using a conventional electroless plating process. For example, when using conventional electroless plating techniques on a bond pad having a cross-sectional dimension of less than about fifty micrometers (50 μm), it may not be possible to form a Ni material having a thickness greater than a few monolayers. Accordingly, methods of electrolessly plating Ni material on bond pads are desired.